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[hfsig] Performance of 24/192 soundcards? USB Soundcards

Thomas Sailer t.sailer at alumni.ethz.ch
Tue Sep 28 09:01:10 UTC 2004


On Mon, 2004-09-27 at 21:06, Anthony.N.Martin at marconiselenia.com wrote:

> A bigger problem I've just thought of is the hard audio bandwidth
> limits applied by filters in many of the CTCSS chips; 300Hz to 3kHz.

Some rigs have -3dB frequency as low as 1kHz, DF9IC did some
measurements about 10 years ago. But that's not that much of a problem,
it's just a linear distortion, so techniques to overcome this are well
known.

> Well that's just broken. You couldn't even use that with a headset
> in your car. I've seen some very cheap PMR446 and CB handhelds and
> I've not seen this. Return to supplier. Problem solved. Next.

No, problem far from solved. What do you want? I was interested in a
faster than 1.2kBit/s robust modulation that worked with the vast
majority of the fielded handheld transceivers. Now you just sent back
standard and yaesu (vertex) and possibly others back to their
manufacturer, at which point you've sent back half or more of the
fielded handhelds. At this point I completely lost interest.

> What did you try?

After I realised that I could either operate in the roughly linear
region, where my modulation signal is drowned by microphone noise, or
drive it hard into compression, I chose the latter. Then there is no
point in even trying to put some information into the amplitude, so I
experimented with various PSK variants. Some remains of that code should
still be in soundmodem. It has some rough edges, I never productized it.

> I suppose that if we want to develop higher-BW more spectrum-efficient
> modulations for V/UHF 12kHz channels we have to design our own digital
> I/Q transceiver.

Seconded.

Tom





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